Zircon Structure

Zircon (ZrSiO4) is the principal repository for zirconium. It has
a fairly simple structure. It has a tetragonal unit cell with a = 6.6 Angstrom
units and c = 6. Zircon is perhaps the most important tetragonal mineral.
Zirconium atoms are at the corners of the unit cell, the center, and at
altitudes of 1/4c and 3/4 c along the midlines of each vertical [001] face.
Silica tetrahedra are centered at top and bottom center, the midpoints of each
edge parallel to c, and at 3/4c and 1/4 c along the midlines of each vertical
face.

The zircon structure seen from the top, looking along
the c axis. Silica tetrahedra are in blue, zirconium atoms in orange, with
darker hues for increasing distance. The coordination polyhedron for the
central zirconium atom is shown in purple.

The zircon structure seen from the side, looking
perpendicular to the c axis. Silica tetrahedra are in blue, zirconium
atoms in orange, with darker hues for increasing distance. The silica
tetrahedra are slightly skewed for perspective, but are really symmetrical
about the faces. Silicon atoms are in green

.The coordination polyhedron for the central zirconium atom is shown in
purple. The central zirconium atom, which is inside the purple
polyhedron, is outlined in gray.

The one somewhat odd feature of this structure is the coordination polyhedron
of the zirconium atoms. The zirconium atoms are surrounded by eight oxygens, two
sets of four at slightly different distances, but is not a slightly distorted
cube. It is very nearly a shape called a snub disphenoid or Siamese
dodecahedron. The ideal shape has 12 equilateral triangle faces and tetragonal
disphenoid symmetry. One way to picture it as a very flat disphenoid cut into
two pairs of faces and pulled apart along the symmetry axis, with a zigzag band
of 8 triangles added in between. Another way is to imagine gluing two pentagonal
pyramids together base to base along three of the five edges, then squeezing
from the side and adding two more triangles to fill the opening.